The combining of cellulose-based materials with plastics was originally developed over 25 years ago. The materials for this process were pre-mixed and cram-fed. Specialist machines have recently been developed, generally based on traditional plastics manufacturing technologies including extrusion and injection moulding. The plastics used include polypropylene (PP), polyethylene (PE) and poly-vinyl-chloride (PVC) and the fillers used include wood flour, flax, jute and other cellulose-based fibre fillers. The more cellulose-based material that is added to the plastic, often the lower the price and, often, the higher the stiffness of the wood-plastic “raw” material. The composite products made from these wood-plastics can generally be nailed, painted and otherwise treated as wood whilst potentially retaining many of the benefits of plastics in the areas of fungal and corrosion resistance.
The introduction of low bulk density natural or wood fibres into extruders or injection moulders or other plastics processing machinery, in particular in a metered or measured way, which is important to achieve desired fibre volume fractions in compositions, has a number of difficulties. It is not straightforward, due to the inherent high volume/low mass nature of such fibres, the lack of free flowing characteristics in such fibres, and fibre bundling or entanglements, to achieve controlled feeding directly into port holes or orifices of plastics processing machinery. Although some fibre-feeders exist they can be either expensive and/or unreliable or inaccurate in metering uniformly over a wide range of fibre feed ratios with wood and other natural fibres. In addition, it is necessary to pre-dry a high volume-low mass of fibre before such feeding/use since such fibres are hygroscopic and retain, or reabsorb, high levels of water, which is usually undesired and required to be substantially removed prior to the plastic processing.
International patent application PCT/NZ2005/000140 discloses a process for producing pellets or granules (as herein defined) comprising fibres of a lignocellulosic material or natural fibres, for use as a feedstock in plastics manufacture, which comprises conveying fibres, produced by mechanically or thermomechanically or chemo-thermomechanically or chemo-mechanically breaking down a lignocellulosic material, or natural fibres, in a dry or wet air stream and applying to the fibres while so conveying the fibres a liquid formulation comprising one or more polymers, monomers, or oligomers, forming the fibres into a solid product, and breaking down the solid product to produce the wood-plastics composite pellets or granules useful as a feedstock in plastics manufacture.